1. Field of the Invention
The present invention relates to interface circuits for receiving high data rate signals from long lengths of cable, and in particular, interface circuits for receiving high data rate, baseband, binary encoded data signals from long lengths of cable.
2. Description of the Related Art
Recovering data which has been transmitted over a long length of cable at high rates requires that such data be equalized in order to compensate for the loss and phase dispersion of the cable. Further, in those applications where the cable length may vary, such equalization must be based upon a complementary transfer function which is capable of adapting accordingly since the transfer function of the cable varies with the length of the cable. This equalizing is generally done using three functions: a filter function; a dc restoration and slicing function; and an adaptation control, or servo, function.
The filter function is performed using a complementary (with respect to the complex cable loss characteristic) filter which synthesizes the inverse of the transfer function of the cable. Since the bit error rate (BER) is directly related to jitter, an important performance metric for an equalizer is jitter within the output waveform. The extent to which the equalizer is able to match the inverse of the complex cable loss characteristic determines the extent to which inter-symbol interference induced jitter is eliminated.
Conventional equalizers use gm/C types of continuous time filters or finite impulse response (FIR) filters. However, these types of filter structures tend to be complex and have difficulty maintaining the required balance among the desired operating characteristics, such as output jitter, compensation for process and temperature variations, and optimization of the signal-to-noise ratio (SNR).
As for the dc restoration and slicing function, referring to FIG. 1, it is well known that ac coupling a digital data stream with variations in pattern density creates baseline wander. For example, if the waveform labelled `A` is presented to the input of the RC ac coupling network, then the waveform labelled `B` results. If the waveform has finite rise times, then the baseline wander will cause jitter in the output. The output jitter arises because a comparator connected to `B` will slice the data at different amplitude points along the waveform edges, and the finite rise and fall times of such edges translate the amplitude slicing variations to timing variations.
Referring to FIG. 2, one conventional technique for eliminating baseline wander is to use a quantized feedback circuit as shown. The simple architecture of such a circuit provides positive feedback around the comparator so that very little charging current flows through the ac coupling capacitor. In other words, the comparator provides its own dc restoration.
Referring to FIG. 3, there is, however, a start-up problem associated with the use of quantized feedback. For example, if the comparator starts out in a state opposite that of the input state, then the output may never transition between states at all because the ac coupled input never crosses the comparator threshold. This situation is aggravated by sparse patterns of the type as shown in FIG. 3.
As for the adaptation control, or servo, function, conventional adaptive equalizers use a peak detection technique in which a control voltage is produced which is proportional to the pulse height of the equalized data signal. However, such a peak detection servo is sensitive to amplitude errors in the incoming data signal.